I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Introduction

Appendicular skeleton

This lecture is an introduction to the events in limb development.

The limb has long been used as a model of how developmental patterning occurs by manipulation of the limb in animal models. This lecture will therefore also introduce some concepts and experiments that have identified patterning mechanisms within the limb.

The previous lecture covered the basics of bone, muscle and cartilage development, that can be applied to the same elements within the limb.

Cells of the ectoderm, cells derived from the dermatome and the hypaxial portion of the myotome mix with somatic component of the lateral plate mesoderm to give rise to the fore and hind limbs.

Limb Buds

One the first noticeable changes is the development of a large blood vessel (marginal vein) which runs just underneath a thickening of the ectoderm at the tip of the limb bud called the Apical Ectodermal Ridge (AER).

Upper and Lower Limb

Human Limb Development during week 6

Human Limb Development during week 8

Limb development occurs at different times for forelimbs and hindlimbs. In the mid-4th week, human upper limb buds first form and lower limbs about 2 days later. The limbs form at vertebra segmental levels C5-C8 (upper limbs) L3-L5 (lower limbs).

Limb Axis Formation

Limbs 28 days

Four Concepts - much of the work has been carried out using the chicken and more recently the mouse model of development.

Cellular origins of the limb

Limb cartilage and bone

Derived from local proliferating mesenchyme derived from the somatic lateral plate mesoderm (somatopleure)

BMP2 and BMP4 play crucial roles in the development of cartilage - sufficient BMP must be present to achieve chondrogenesis. However, the main role is in later bone formation. Loss of BMP2 and 4 leads to a severe impairment of osteogenesis

Maternal

Genetic

Trisomy 21 - Downs syndrome

Human Gene Mutations - mutation of any of the patterning genes will result in limb abnormalities

Type II syndactyly- HoxD13

Syndactyly

Syndactyly

Fusion of fingers or toes (Greek, syn = together, dactyly = digit) which may be single or multiple and may affect: skin only, skin and soft tissues or skin, soft tissues and bone. The condition is unimportant in toes but disabling in fingers and requires operative separation and is frequently inherited as an autosomal dominant. The presence of this additional "webbing" reflects preservation of the developmental tissues that in normal development are removed by programmed cell death (apotosis).

Talipes Equinovarus

Talipes equinovarus

(Latin, talipes = ankle bone, pes = foot, equinus = horse) Abnormality of the lower limb which begins in the embryonic period (first trimester of pregnancy) resulting in the foot is then turned inward and downward at birth, described as "club foot". Occurs in approximately 1 in 1,000 births, postnatally it affects how children walk on their toes with the foot pointed downward like a horse.

Fig. 251. Showing the Origin of the Ligamentum Teres and Reflected Bundle of the Capsular Ligament.

Fig. 252. Showing the Origin of the Crucial Ligaments of the Knee.

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